Field of the Invention
The invention relates to a radio frequency technique, and particularly relates to a transmitter, a common mode transceiver using the transmitter and an operating method thereof.
Description of Related Art
In a communication system, if a distance between two communication equipment is very far away, a problem that the two equipment have different ground potentials is encountered. According to a commonly used conventional technique, there are two solutions for resolving the above problem. Referring to FIG. 1 for the first conventional solution, FIG. 1 is a structural diagram of ground potential connection between the two communication equipment in the communication system. Besides that a transmission interface between the two communication equipment EQ1, EQ2 is used for transmitting signals (for example, the transmitted signals include an interface voltage Vbus), ground potential signal pins of the two communication equipment are connected to achieve a same ground potential GND. However, such solution has two disadvantages, and the first disadvantage is that a connection port of the communication equipment is required to use the ground potential signal pin, and the second disadvantage is that when the ground potential between the two communication equipment EQ1 and EQ2 is excessively large, when the different ground potentials are forced to be connected, since a resistance of a connection line is in a micro-ohm level, a large current (I=V/R) is probably generated, and the large current is bound to produce a magnetic field to influence a normal signal communication, which may cause communication failure.
Referring to FIG. 2 for the second conventional solution, FIG. 2 is a structural diagram of the two communication equipment in the communication system having a common mode range interface. The communication equipment EQ1 and EQ2 are implemented by common mode transceivers, and in a common mode voltage range, the communication equipment EQ1 and EQ2 are allowed to have different ground potentials.
Referring to FIG. 3, FIG. 3 is a structural diagram based on FIG. 2, which illustrates a positive ground potential offset between the communication equipment EQ1 and EQ2. For example, when the common mode voltage range of the communication equipment EQ1 is 0V to 5V, and the positive ground potential offset is 7V, a demand of the common mode voltage range of the communication equipment EQ2 is 7V to 12V.
Moreover, referring to FIG. 4, FIG. 4 is a structural diagram based on FIG. 2, which illustrates a negative ground potential offset between the communication equipment EQ1 and EQ2. For example, when the common mode voltage range of the communication equipment EQ1 is 0V to 5V, and the negative ground potential offset is −7V, a demand of the common mode voltage range of the communication equipment EQ2 is −7V to −2V. On the other hand, if the communication equipment EQ2 is required to simultaneously deal with the positive ground potential offset and the negative ground potential offset, a demand of the common mode voltage range thereof is −7V to +12V.
When the communication equipment EQ1 and EQ2 are all common mode transceivers, and the positive ground potential offset or the negative ground potential offset exists between the two communication equipment EQ1 and EQ2, according to related regulation, signals are required to be normally transceived within the ground potential offset range and additional leakage is not allowed. Referring to FIG. 5 or FIG. 6. FIG. 5 is a schematic diagram of a leakage path when a transmitter of the common mode transceiver has the positive ground potential offset. FIG. 6 is a schematic diagram of a leakage path when the transmitter of the common mode transceiver has the negative ground potential offset. Generally, a pull-up circuit 11 or a pull-down circuit 12 of the transmitter 10 of the transceiver all have a parasitic diode.
In FIG. 5, when the interface voltage Vbus is greater than a power voltage VDD, a leakage path where current flows from the interface voltage Vbus to the power voltage VDD through the parasitic diode is formed. Similarly, in FIG. 6, when the interface voltage Vbus is smaller than the ground potential GND, a leakage path where current flows from the ground potential GND to the interface voltage Vbus through the parasitic diode is formed.
In order to resolve the problem of the leakage path, referring to FIG. 7, FIG. 7 is a circuit diagram illustrating a transmitter of a conventional common mode transceiver. A reverse diode 71 is disposed on a path where current flows from the interface voltage Vbus to the power voltage VDD for connecting the pull-up circuit in series, so as to block the leakage path of the positive ground potential offset; and another reverse diode 72 is disposed on a path where current flows from the ground potential GND to the interface voltage Vbus for connecting the pull-down circuit in series, so as to block the leakage path of the negative ground potential offset. However, the above solution has a disadvantage that an upper limit and a lower limit of the common mode voltage range are respectively decreased by a cut in voltage of one diode, so that output capability of the transmitter 70 is deteriorated. For example, before the two reverse diodes are configured, the common mode voltage range is 0 to VDD, and after the two reverse diodes are configured, the common mode voltage range is decreased to 0.7 to VDD-0.7.